1. Field of the Invention
This invention relates to a color cathode ray tube and more particularly to a color cathode ray tube including an automatic convergence unit employing index phosphor elements on the reverse surface of a color-selective electrode.
2. Description of the Prior Art
In cathode ray tubes, three cathode rays or electron beams are usually employed, these rays or beams being emitted from electron guns and transmitted to three types of phosphors R, G and B provided on the display surface. Unless each of these rays or beams is converged on one predetermined point on the color-selective electrode, a so-called color shift occurs.
In color cathode ray tubes employed for high accuracy display on a so-called color monitor, it has been suggested to provide a so-called automatic convergence unit, according to which, for preventing the aforementioned color shift, the sweeping position of the cathode rays is corrected on the basis of the light emitted when the cathode rays sweep index elements formed on the reverse surface of the color-selective electrode facing to the electron gun.
These cathode ray tubes are provided with index elements formed by coating index phosphors on the surface of the color-selective electrode facing to the electron gun with a predetermined pattern configuration, as proposed in the Japanese Patent Applications KOKAI Nos. 58-24186, 58-25042 and 61-156623.
For forming these index elements, the aforementioned reverse surface of the color-selective electrode is masked with a predetermined pattern configuration and spray-coated with a paste containing index phosphors, such as P46 or P47. Such paste may include the aforementioned index phosphors as a mixture with a binder such as nitrocellulose dissolved in an organic solvent system.
When the paste coated as described above for forming the index element is dried in situ, the organic solvent is evaporated so that there remain only the index phosphors and the binder on the electrode surface. Since the index phosphors are affixed to the color-selective electrode by the binder with only a weak force, they are peeled off when incidentally contacted by hands or fingers. The index elements may also be peeled off when air blowing, for example, is resorted to for removing the index phosphors affixed to the slits formed on the color-selective electrode. In addition, when the color cathode ray tube in its entirety, inclusive of the aforementioned paste, is heated to a temperature of 380.degree. to 450.degree. C., during a step following the coating, such as for baking or frit sealing the tube bulb, the binder is burned and scattered, so that only the index phosphors remain on the index elements. Since the index phosphors do not have the intrinsic adhesive force, the index elements may be peeled off easily with vibration or shock.
When the index elements are peeled off, the signal output level resulting from the light emitted from the index elements is lowered so that the sweeping positions of the cathode rays cannot be corrected accurately. Also, when the index phosphors thus peeled off are affixed, for example, to the electron gun, electrical discharge may occur to render it difficult to produce a high quality image or picture.
Since the organic solvent is contained in the paste, attention is required to maintain the plant in a ventilated condition by provision of a special ventilator, with the result that the paste coating operation cannot be performed easily and economically.